1. Field of the Invention
The present invention relates to cationically charge-modified membranes and processes to prepare such membranes. Cationic charge-modified membranes disclosed herein are produced via post-treatment of formed hydrophilic or hydrophobic membranes or are produced from membrane casting processes incorporating cationic components.
2. Background of the Technology
Cationically charge-modified membranes are useful in the removal of a variety of materials from solutions and also in certain biotechnological applications. For example, positively charged membranes are useful in the removal of endotoxins from solutions. Endotoxins are toxic substances often derived from bacterial lysates. In addition, such membranes have found utility in the removal of negatively charged species from feed-streams, such as in the preparation of ultrapure water for the semiconductor industry.
Ultrafiltration and microfiltration membranes utilized in industry, particularly in the food processing industry and in environmental applications, are typically hydrophobic membranes which may be surface-modified with a hydrophilic material to reduce fouling and to confer additional desirable properties to the membrane. Membranes may be isotropic or asymmetric (anisotropic) in their pore structure. Isotropic membranes have a uniform pore structure throughout the membrane. Asymmetric membranes do not have a uniform pore structure throughout the membrane. Asymmetric porous membranes are distinguished from isotropic, homogeneous membrane structures whose flow and retention properties are independent of flow direction. Asymmetric membranes are useful in microfiltration, ultrafiltration, and reverse osmosis processes.
Several different processes and reagents have been utilized to produce cationically charge-modified, initially hydrophilic or hydrophobic membranes, and related membranes.
U.S. Pat. No. 4,012,324 to Gregor discloses casting formulations including a matrix polymer, a polyelectrolyte, a solvent, and a chemical cross-linking agent. Membranes are formed therefrom through a process of evaporating the solvent to form a membrane of uniform porosity and macroscopic homogeneity, having fixed anionic or cationic charges and a water content of from about 15 to about 75%. Membranes with substantial equilibrium water content are known as hydrogels and are subject to loss of water unless protected prior to use and additionally have limited application.
U.S. Pat. No. 4,673,504 to Ostreicher, et al., discloses cationic charge-modified microporous membranes that are produced from hydrophilic organic polymeric microporous membranes. These microporous membranes are hydrophilic and isotropic, with uniform pore structure throughout the membrane. All hydrophobic membranes as well as anisotropic hydrophilic membranes are excluded from the '504 patent.
In U.S. Pat. Nos. 4,737,291 and 4,743,418 to Barnes, Jr., et al., hydrophilic membranes which absorb or adsorb water and which contain hydroxyl, carboxyl or amino substituents are cationically modified and chemically crosslinked. Again, all hydrophobic membranes are excluded from the '291 and '418 patents.
U.S. Pat. No. 4,797,187 to Davis, et al., discloses a method to prepare ionically bonded coacervate layer membranes having improved selectivity. However, the invention of the '187 patent does not produce cationically charge-modified membranes as final products.
In U.S. Pat. No. 5,004,543 to Pluskal et al., hydrophobic substrates, comprising hydrophobic, microporous membranes having a crosslinked, cationic charge-modified coating are disclosed. The microporous membranes of the '543 patent are substantially isotropic, with a pore structure that is substantially uniform throughout the membrane. Charge-modified, hydrophobic membrane materials as disclosed in the '543 patent do not wet instantly when immersed into aqueous solutions. Monomeric, lower molecular weight wetting agents such as those utilized in the '543 patent are generally less efficient than higher molecular weight polymeric wetting agents for surface wetting, due to the production of much thinner surface adsorbed film layers and often also due to the production of layers with incomplete surface coverage.
U.S. Pat. No. 5,098,569 to Stedronsky discloses surface-modified polymeric support membranes and a process for preparing a surface-modified membrane. Membranes are preferably hydrophobic and are surface-modified via the irreversible adsorption of a monomolecular layer of an activated modifying polymer. The Stedronsky patent employs a multistep process for preparing a surface modified membrane which consists of a polysaccharide, chemically crosslinked, modified surface which is not cationic.
U.S. Pat. No. 5,151,189 to Hu, et al., discloses cationic charge-modified microporous hydrophilic membranes, as well as preparation of the same by post-treatment. The post-treatment process of the '189 patent begins with an inherently hydrophobic membrane made hydrophilic during manufacture with an intrinsic wetting agent such as polyvinylpyrrolidone or polyethylene glycol. It is desirable from the perspective of simplicity to avoid additives such as PVP and PEG which rely upon latently reactive functional groups to enhance hydrophilicity. Additionally, PVP and PEG are known to have lower surface adhesion to selected hydrophobic surfaces than other polymers with hydrophilic functional groups.
U.S. Pat. No. 5,282,971 to Degen, et al., discloses a filter medium comprising microporous polyvinylidene fluoride membrane and a polymer containing positively charged quaternary ammonium groups covalently bonded to the membrane, and a method of using the membrane. The '971 patent relies upon careful control of polymerization reagents and a polymerization process, most often employing ionizing radiation, to produce a final membrane with the desired properties.
U.S. Pat. No. 5,531,893 to Hu, et al., discloses a hydrophilic charged modified microporous membrane having a crosslinked structure of an interpenetrating polymer network. The membrane comprises a homogeneous matrix of polyether sulfone (PES), polyfunctional glycidyl ether, and a polymeric amine such as polyethyleneimine (PEI) and like polyamines, and polyethylene glycol. A shortcoming of the '893 patent is that membranes heated for the stabilization of the network structure have a lower cationic charge density. This is stated to be due to gradual decomposition of crosslinked PEI adduct in the membrane structure.
Thus, while it can be seen that various different processes and reagents have been utilized to produce cationically charge-modified membranes, each of the cited references has one or more undesirable features. None of the cited references combines initially hydrophobic membranes with optimal polymeric wetting agents and cationic charge-modifying agents in a simple process to produce a stable, cationically charge-modified, isotropic or anisotropic, optionally non-hydrogel membrane. Additionally, none of the cited references produces stable, cationically charge-modified, isotropic or anisotropic, optionally non-hydrogel membranes in a simple casting process without chemical crosslinking agents. Accordingly, there remains a need for improved, stable, cationically charged membranes which possess a plurality of fixed formal positive charges that can be readily produced from initially hydrophobic membranes or from polymer starting materials in a casting process without complication or expensive apparatus and which are not restricted to isotropic or hydrogel membrane types.